bradfitz@laptop ~$ cat x.go
package main

import "fmt"

type T struct {
        toInt    map[string]int
        toString map[int]string
}

var t = T{
        foo.toInt:    make(map[string]int),
        bar.toString: make(map[int]string),
}

func main() {
        fmt.Printf("Got: %#v\n", t)
}

bradfitz@laptop ~$ GOROOT=$HOME/go1.4 go run x.go
# command-line-arguments
./x.go:11: invalid field name foo.toInt in struct initializer
./x.go:12: invalid field name bar.toString in struct initializer

bradfitz@laptop ~$ GOROOT=$HOME/go1.5 go run x.go
# command-line-arguments
./x.go:11: invalid field name foo.toInt in struct initializer
./x.go:12: invalid field name bar.toString in struct initializer

bradfitz@laptop ~$ GOROOT=$HOME/go1.6 go run x.go
# command-line-arguments
./x.go:11: invalid field name foo.toInt in struct initializer
./x.go:12: invalid field name bar.toString in struct initializer

bradfitz@laptop ~$ GOROOT=$HOME/go go run x.go
Got: main.T{toInt:map[string]int{}, toString:map[int]string{}}

package p

import (
    "errors"
    . "unsafe"
)

type T struct {
    New     int
    Pointer int
}

var t = T{
    errors.New: 1,  // ERROR
    Pointer: 2,     // ok
}

 package p
 import . "fmt"
 type T struct { Println int }
 var t = T{ Println: 0 }